The invention relates generally to an acceleration limit reset for fuel controls of gas turbine engines having acceleration limiters. The invention is more directly pertinent to the reset of the acceleration limit for transient conditions including a "reslam" condition.
The closed loop fuel control of gas turbine engines based on acceleration has become successfully implemented in various systems. These systems act directly to control the rate of change of the engine speed as a function of an acceleration term. The acceleration term is formed by differencing a scheduled term with an actual or an implied actual parameter of the gas generator. The actual acceleration of the engine is fed back through changes in the actual term for comparison with the scheduled term. The acceleration term, which after an integration effectively provides a datum for a proportional speed control loop, may therefore, be a function of any of a number of control input parameters including engine speed, ambient pressure, temperature, compressor pressure, etc. Advantages of this form of acceleration control include consistent predictable accelerations independent of fuel type, temperature, and altitude. The acceleration time is also generally independent of air bleed and power extraction status.
A closed loop fuel control for a gas turbine engine based on acceleration is more fully disclosed in U.S. application Ser. 210,938, filed in the name of Roland Marston Evans on Nov. 28, 1980, which is commonly assigned with the present application. The disclosure of Evans is hereby expressly incorporated by reference herein. Other examples of closed loop systems of this type are illustrated in U.S. Pat. Nos. 4,018,044; 4,100,731; and 4,040,250.
Because the error or acceleration term may exceed the surge capability of the engine, it is conventional in a closed loop control to limit the acceleration term according to an acceleration schedule which defines the surge line of the engine in terms of at least one engine operating parameter. Thus, if a scheduled acceleration term exceeds this limit function at a particular system operating point, the control will regulate the fuel flow accordingly and cause the engine to accelerate at the lower limit value. In many controls, the acceleration schedule is variable with respect to one or several operating parameters of the engine in order to provide an adequate stall margin while maintaining the maximum acceleration limit available over various operating conditions.
There are however, certain special transient conditions where the steady state stall margin is considerably reduced and the acceleration limit should be additionally modified or reset while these conditions exist. Since it is desirable to operate as close to the steady state surge line of the engine as possible it is not advantageous to schedule for these conditions in the normal manner. If a transient margin is included in the normal schedule, then the engine will not be able to accelerate to the full extent available in steady state conditions.
One of the most critical of these special conditions is what is termed a "reslam" operation. The condition is initiated when the engine has been in a steady state condition at a power level near maximum for a substantial period of time. The engine is very hot and therefore, fuel is very efficiently used. If, during this time, the power demand lever is retarded rapidly to idle or a reduced power position but then pushed back to a maximum level before the engine has been allowed to reach a thermal equilibrium, a reslam condition occurs. The reslam condition may produce a reduction in the stall margin of the engine whereby the overfueling capacity or surge margin may be 50% less than during a normal acceleration. It is therefore, imperative that the fuel control not excessively overfuel during this condition.
The reduction of stall margin during a "reslam" condition is due primarily to the thermal inertia of the temperature sensitive components in the engine such as air seals, compressor blades, and the like. Therefore, the stall margin is reduced at the moment of deceleration and the amount of reduction is a function of the difference in power levels. The steady state stall margin will be restored over time as the engine regains thermal equilibrium at its new operating point. If, however, an acceleration is demanded prior to the engine reaching thermal equilibrium, a reset of the acceleration limit should be implemented to prevent stall.
A "reslam" condition is even more critical in a two-spool gas turbine engine where a high pressure compressor spool and a low compressor spool are used in combination. During an extremely rapid deceleration which generally initiates a "reslam" condition, the normal relationship between the speeds of the high pressure compressor spool and low pressure compressor spool can be lost. The compressors have different inertias and if not decelerated at substantially the same rate lose their synchronous relationship because of the difference. The speed difference will cause the low pressure compressor to overboost the high pressure compressor to create a mismatch. If this occurs and a "reslam" acceleration is requested the engine may stall since the surge margin is drastically reduced during the mismatch. The engine must, therefore, be accelerated at a different rate than the steady state acceleration limit until the steady state relationship is regained to prevent stall.